1. Field of the Invention
This invention relates to a cam follower apparatus, specifically, a rolling/sliding member such as the outer ring of a cam follower apparatus in the valve drive mechanism of an engine.
2. Description of the Prior Art
There are many kind of automobile engines including reciprocating piston type engines, all of which except for some 2-cycle engines, have air-intake valves and exhaust valves, which open and close in phase or synchronizing with the rotation of the crank shaft.
A valve drive mechanism is used to transmit the motion of the cam, attached to the cam shaft, to the valves as the cam shaft rotates with the engine's crank shaft.
There are many kinds of valve drive mechanisms for driving the air-intake valves and exhaust valves. For example, in the valve drive mechanism of a SOHC-type engine, as shown in FIG. 3, the single cam shaft rotates at 1/2 the speed of the crank shaft (for a 4-cycle engine), to drive the air intake valve 17 and exhaust valve 18 by way of the rocker arms 16. Specifically, the cams 19 are attached to the cam shaft 15 which rotates in phase with the crank shaft 14, so that they come in contact with the ends of the rocker arms 16 to drive the air-intake valve 17 and exhaust valve 18.
In recent years, it has been widely practiced to equip the valve drive mechanism with a cam follower, so that when the valve drive mechanism is in operation, the friction that occurs is transformed from sliding friction to rolling friction, thus keeping the friction loss to a minimum.
In order to reduce the friction force, which occurs between the edge of the cams 19 and its mating member such as rocker arms 16 while the engine is running, and in order to reduce fuel consumption when the engine is running, it has become common practice to place a cam follower apparatus, at the point of contact, that rotates with the rotation of the cams 19.
This kind of cam follower apparatus has an outer ring which is placed in a rolling and/or sliding contact with the other parts.
As shown in FIG. 4 thru FIG. 6, a pair of supports wall portions 20 with a space therebetween are located at the end of the rocker arm 16 facing to the cam 19, and both ends of a shaft 21 are attached to support wall portions 20. A short, cylindrical shaped outer ring 23 is located around this shaft 21, and comes in contact with the shaft 21 by way of rollers 22, or comes in contact with the shaft 21 directly. The outer peripheral surface of this outer ring 23 comes into contact with the outer peripheral surface of cam 19, so that as cam 19 rotates, the outer ring 23 rotates around the shaft 21.
By using this kind of outer ring 23, the friction, between the cam 19 and the parts that come in contact with it, is changed from sliding friction to rolling friction, thus lowering the fuel consumption rate.
When this kind of cam follower apparatus is installed in the valve drive mechanism of an engine, however, damage, such as pitting, easily occurs on the outer peripheral surface of the cam 19, caused by the fluctuating load that is applied to the outer peripheral surface of the cam 19 by the outer peripheral surface of the outer ring 23.
When the rolling/sliding member is used in severe lubrication conditions, only the ends of very minute protrusions formed on the surface of the member come in direct contact (metallic contact) with the surface of the mating parts without a film of oil between them. In this case, most of the load applied to the rolling/sliding member is supported by the load with several minute protrusions which come in contact with the opposition parts, so that large stress concentrations, together with tangential forces are applied to each minute protrusion.
Due to the stress concentrations and tangential forces, small cracks occur on the surface of the rolling/sliding member, and as these cracks advance, peeling occurs.
In order to prevent friction, the surface of the rolling/sliding member could be made so harder, but, by increasing the hardness in such a degree, cracks are more easily caused by the high stress concentrations, which leads to the peeling.
In order to improve the durability of the surface of the rolling and/or sliding member, many inventions have been proposed in the past with respect to the improvement of the surface condition.
As was described on page 39 of the special issue, "Roller Rocker Arm and the Reduction of Friction", in the July, 1989 edition of the magazine, "Automotive Engineering(Japan)", the cam shaft 15 including the cam 19, is made of very strong and hard metal material, such as, hardened cast iron, chilled cast iron, hardened steel, or a sintered alloy, that can withstand pitting.
Japanese Patent Publication No. H1-30008, discloses an invention relating to a bearing roller having a rolling surface of which the surface roughness, Rmax, is between 0.3 to 1.5 .mu.m, and which is formed with scratches in random directions, and a surface layer having a residual stress of 50 kgf/mm.sup.2 or more.
Also, Japanese Patent First Publications No. H3-117723 to No. H3-117725, discloses an invention relating to forming several depressions at random on the rolling surface using barrel processing, and in addition making the hardness of the surface layer harder than the inside or bulk hardness, such that residual compressive stress is caused to occur in the surface layer.
Moreover, Japanese Patent First Publication No. H3-199716 discloses a bearing, where the surface that contacts other parts is hardened using a surface hardening process, and where the depth of the peak value of the residual compression stress is equal to the depth of the peak value of the shear stress distribution.
Also, Japanese Patent First Publication No. H4-54312 discloses an invention relating to bearing parts, where the residual compressive stress, formed by using a shot peening process, is 100 kgf/mm.sup.2 or more on the surface, and the stress is 40 kgf/mm.sup.2 or more at 300 .mu.m below the surface.
Japanese Patent Publication No. H2-17607 discloses a surface processing method where shots are provided in the range of 40 to 200 .mu.m is size to have a hardness equal or higher than the hardness of the product, and the shots are projected over the surface of the product at a speed equal to or higher than 100 m/sec so as to increase the temperature around the surface higher than the As transformation point.
In these aforementioned inventions, depending on the operating conditions, excellent durability is obtainable. However, when these inventions are applied to the rolling and/or sliding members which are used in conditions of poor lubrication in a rolling and/or sliding contact with other parts, it is impossible to obtain sufficient durability.
For example, when these inventions are applied to a cam follower apparatus which has an outer ring in a rolling and/or sliding contact relationship with the outer peripheral surface of the cam while the engine is running, peeling may occur, shortening the life of the follower.
In the case where the cam shaft 15 including the cam 15 is made of very strong and hard metal material such as the quench-hardened cast iron as in "Automotive Engineering(Japan)" as mentioned previously, the peeling is inclined to occur on the outer peripheral surface of the outer ring 23 in contact with the cam 19.
Specifically, it is difficult to finish the outer peripheral surface of the cam 19 made of a hard metal material such as the quench-hardened cast iron, and a lot of minute protrusions are formed on the outer peripheral surface of the cam 19, as shown in FIG. 8, which has been subjected to the normal industrial surface finishing method. The surface roughness of the outer peripheral surface is relatively rough, that is about 0.4 .mu.m Ra to 0.8 .mu.m Ra.
On the other hand, the outer ring 23 is made of bearing steel which is harder than the cam 19, and usually subjected to the superfinish on its surface because the outer ring 23 has a simpler shape and is easier to be processed than the cam shaft 15 including the cam 19.
Accordingly, the outer peripheral surface of the outer ring 23 is smoothly finished as shown in FIG. 7 and FIG. 8 with the surface roughness around 0.05 .mu.m Ra, so that the outer peripheral surface of the outer ring 23 has a limited amount of lubricant 24 on it, which may lead to the occurrence of peeling when the outer ring 23 is installed in a cam follower apparatus and experiences rolling/sliding contact with the outer peripheral surface of the cam during the engine running.
In OHC-type and DOHC-type engines, where the valve drive mechanism is placed on top of the engine, the supply of lubricating oil to the valve drive mechanism is insufficient, so that when the engine is running, the lubrication condition becomes very severe. In this kind of condition, when a rolling and/or sliding contact relationship is provided between the outer peripheral surface of the outer ring and the outer peripheral surface of the cam, 5 to 10 .mu.m deep peeling usually occurs on the outer peripheral surface, so that the life of the outer ring is shortened.
Of the technology mentioned above, in the invention described in Japanese Patent First Publication No. H4-54312, if the lubrication condition is severe, regardless of the friction conditions, peeling cannot be sufficiently prevented.
Also, in the invention described in Japanese Patent First Publication No. H3-199716, there is no real problem when the friction is only rolling friction, however, when sliding friction occurs, peeling cannot be sufficiently prevented.
Moreover, in the inventions described in Japanese Patent Publications No. H1-30008, and No. H3-117723 to No. H3-117725, if the lubrication condition is severe, and if the operating conditions are severe, such as a large contact load, again peeling cannot be sufficiently prevented.
The bearing part of Patent Publication No. H2-17607 can not afford to provide enough anti-peering property if no additional process is added.